An early arrangement for the flat packaging of semiconductors in an assembly of two or more such packages has been well known for some time. Opposed end plates and a heat sink housing between which are mounted two flat package semiconductor devices are held together by a tie rod and nut is shown in U.S. Pat. No. 3,551,758. In an effort to achieve a uniform force loading on each of the semiconductor devices, truncated cone shaped spring washers were arranged face to face or base to base in cavities in one of the end plates. A bearing plate in contact with the spring washers has some flexibility and is designed to assist in distributing the force loading uniformly over a bottom surface of a flexible portion of the housing and to permit effective levelling of the semiconductor devices. However, such an arrangement cannot assure that the pressure will be equal on the several wafers in a package, particularly if the wafers are not truly parallel or if the wafers are of different sizes. Furthermore, to the extent that wafers within a package have different clamping force requirements, the prior art as exemplified by this patent does not teach or suggest how that can be accomplished easily. Furthermore, the clamping arrangement results in a complex and bulky assembly which is undesirable where space savings and weight are important.
U.S. Pat. No. 3,654,528 shows a semiconductor wafer current controlling device in which a means for cooling a wafer is provided with the object to avoid the need for heavy clamping forces. In this arrangement, the wafers are mounted between two spaced apart ceramic mounting rings, the rings are clamped together under a modest amount of pressure and are surrounded by an insulating housing. This assembly is then mounted between a pair of substantially identical heat sinks comprising tubular outer member of a high conductivity metal which are clamped together by a plurality of elongated studs which utilize resilient washers. A small space is left between each end face of the heat sink members and an adjacent flange of sheet metal sealing members so as to contain liquid metal under pressure. The liquid metal heat from the wafer in passing radially outwardly along the lower face of the wafer and downwardly through an annular passage where the heat is released to the tubular heat sink member through the surface of a bore therein. After having released the heat extracted from the wafer, relatively cool liquid enters a channel extending across the bottom face of a heat sink core 60 and is again fed back to extract heat from the wafer. Although the purpose of the clamping studs is to substantially equalize fluid pressures developed in fluid passages on opposite sides of the wafer, the pressure is kept substantially equal by pumps. It will be readily appreciated, however, that such an arrangement is extremely complex, heavy and massive and does not provide for the simplicity and space-saving that is needed in applications where space is at a premium.
Another arrangement for holding a semiconductor element is shown in U.S. Pat. No. 3,789,248. This arrangement is concerned with the application of dynamic mechanical stresses which increase pressure and would otherwise exceed acceptable limits for the semiconductor elements, thereby leading to their destruction. To address this problem, pressure plates are provided which have convex or dome-shaped projections directed toward support plates. To establish clamping pressure, a pair of bolts extend between and through the spaced support plates. The surfaces of the support plates directed toward the dome-shaped configuration of the pressure plates are provided with a titanium carbide coating which does not undergo any substantial plastic deformation due to clamping action. As a result of such an arrangement, pressure increases due to dynamic mechanical stress remain within certain harmless limits and the clamped semiconductor assembly within the support plates is displaced symmetrically so that kinetic energy developed in dynamic mechanical stresses is transformed into harmless frictional heat. Although such a holder is relatively simple in construction, it is not particularly adapted for an integrated package wherein several different types of wafers may be included which require different clamping forces. Furthermore, it does not take into account that the component faces may not be parallel because of manufacturing tolerances.
U.S. Pat. No. 4,138,692 is primarily a cooling arrangement which uses fluid to cool but not to clamp a chip. A resilient bellows is provided for the purpose of adjusting to dimensional variations of each chip so that a relatively flat surface engagement can be achieved. Although such an arrangement may be effective as a cooling system, it does not provide for positive clamping of a semiconductor wafer. Coolant such as helium gas which is provided between a cap and a flange from the substrate which defines a hermetic chamber is for the purpose of cooling and not clamping. Such an arrangement cannot provide the positive compression forces required for large semiconductor packages.
The concept of an integrated electronics package is shown in U.S. Pat. No. 4,313,128. For example, a rectifier and thyristor may be enclosed within the same case. The package is held between a first bar member and a second bar member which are compressed by tightening bolts passing through the bars to apply a compressive force. This arrangement contains the disadvantage that it does not necessarily apply compressive forces to each of the wafers in the package uniformly and may well exceed the recommended compressive force for a particular wafer.
The clamping means disclosed in U.S. Pat. No. 4,392,153, shows a thyristor held between two blocks to which heat sinks are fastened. A diffusion bonding press forming thermo-compression diffusion bonds in the semiconductor device comprises an upper metallic plate parallel to a lower metallic plate which are joined by bolts so that a thermo-compression diffusion bond can be achieved between a heat sink and a copper strain buffer placed in the diffusion bonding press and heated. However, this press is not intended as part of the final assembly and does not deal with the problem of uniformity of forces on the thyristor components.
U.S. Pat. No. 4,402,004, discloses a transistor element mounted on a supporting plate. An upper disc and a lower disc press toward each other with the semiconductor element disposed therebetween so as to establish electrical contact. However, this arrangement is not concerned with the problems arising from variations in manufacturing tolerances as well as using different semiconductor elements which require different clamping forces.
Another pressure-applied type semiconductor device is shown in U.S. Pat. No. 4,500,907. A semiconductor body has a metal stamp provided on one side thereof along with a thermal compensation plate. The metal stamp is provided with an annular peripheral groove or other type of groove so that the groove is deformed when compressive load is applied to the semiconductor body. This is intended to alleviate local stress concentration in the semiconductor body as in the area of the body right beneath the edge of the metal stamp. While such an arrangement may be useful for preventing point application of stresses, it is not helpful where several different types of wafers are to be used and a simple arrangement is needed to apply a uniform clamping force in a relatively small package.
A clamping arrangement proposed for integrated wafer packages is shown in U.S. Pat. No. 4,739,447. In that arrangement, there is provided a molded frame having a recess in which is fitted a unilateral heat sink. A clamping device is associated with each of the semiconductors through a common connecting bar. Although this arrangement is intended to accommodate semiconductor power components of varying heights in a package and to facilitate the mounting and connection of these components, it requires individual clamping devices for each component and thus unduly complicates the package arrangement.
German Offenlegungsschrift No. 2805771 discloses a chip mounted in a liquid-type metal case which is under pressure from a cooling medium. The chip is pressed to at least one wall of the case by the cooling medium. This arrangement applies uniform pressure across the chip only if the pressing plate forced against the chip by the coolant pressure is true and contacts the chip uniformly across its surface. Moreover, a separate arrangement would have to be provided for each chip in a package which would result in a complex package arrangement.
German Offenlegungsschrift No. 3342102 shows a semiconductor wafer clamped between triangular heat dissipating plates which are clamped together by three axial bolts. Each of the plates carries a pattern of projections which form contact surfaces and spacer which leave room for circulating cooling fluid. This arrangement does not solve the problems arising from non-uniform compression forces and does not provide simplicity needed in an integrated wafer package.
Power bars employ large power semiconductors, i.e. a diameter in excess of 15 mm, which are mounted to the board by use of compressive force rather than by soldering for making electrical and thermal connections. Consequently, manufacturers of power semiconductors require that the surface to which the semiconductor is fixed be extremely flat. Compressive forces on the semiconductor must be within a range specified by the manufacturer and also be evenly distributed over the surface of the semiconductor. There is often a need to fix a heat sink to the top of each of the semiconductors. The bottom surface of a heat sink would also have to be extremely even to be pressed against the semiconductor surface within the tolerance range.
Manufacturers of large wafers recommend several methods for mounting which result in "hockey puck" power transistors and thyristors. In one method, the transistor is mounted between two heat sinks having a required surface flatness of 0.0005 in. to 0.001 in. maximum between the highest and lowest points in the area where the device sits.
Another technique requires a peak-to-trough of 30-40 .mu.m in the seating area. In either of the aforementioned mounting techniques, however, it is absolutely essential that the compression or clamping force be equally distributed across the device contact faces in order to ensure that good electrical and thermal contacts are made between the external heat sinks and between the wafer itself and the contact areas internally of the wafer. The latter is made possible by allowing large metal contacts on the top and bottom of the wafer in the "hockey puck" to move slightly relative to the ceramic sides of the wafer.
Such "hockey puck" packages are relatively large and heavy. Consequently, such packages are unsuitable in applications where size and weight must be minimized such as, for example, in aircraft applications. Accordingly, it has been recognized that to minimize the size and weight of an electronics assembly, the wafers used in the hockey puck wafer package must be put into smaller and lighter packages. Nevertheless, the wafer-to-contact interface must nevertheless remain under compression as specified by the wafer manufacturers.
Users of such wafer packages also desire to package as many wafers for an electronic assembly in a single package to shorten electrical interconnections. Such an integrated electronics package is then clamped to a heat sink/exchanger to cool the semiconductors in the package.
However, an integrated electronics package containing different components has problems. One problem arises from the fact that the similar wafers which are not uniform in diameter, thickness or clamping force requirements are intended to be used in the same package. For instance, the package designated generally by the numeral 10 in FIGS. 1 and 2 utilizes three transistors 11,12,13 and two diodes 14,15 integrated in a thin flat package 16. In order to meet the clamping force specifications on each wafer when this package is clamped to a heat sink, the surfaces of the heat sink, clamp and electronic package must be perfectly flat and parallel to assure that the compressive force will be uniform across the top and bottom faces 17,18 of the package. However, normal manufacturing tolerances alone will often prevent achieving a truly flat and parallel top and bottom faces. In that instance, the compressive force on the package will not be uniform and the semiconductors will not be evenly clamped to their specified values as shown schematically in FIG. 3 where the right side of the top surface 17 is higher than the left side causing greater concentration of forces on the components 14,15 and less force than desired on the transistor 12.
Disclosure Of Invention
The foregoing problems have been overcome in accordance with the present invention with the recognition that pressure in a static fluid is equal in all directions regardless of the fluid boundary shape. This principle has been utilized in the present invention by providing a pressurized fluid layer in a flexible bladder which is inserted between external clamp plates and a top or bottom surface of the electronics package or, alternatively, between the electronics package top and the electrical contacts. The foregoing arrangement allows the fluid layer to compensate for any irregularities in flatness or parallelism while, at the same time, maintaining uniform pressure over the surface of the semiconductor wafer.
An object of the present invention is to provide hydrostatic clamping of wafer packages or power bars in order to achieve a simple and yet effective clamping to assure good electrical contact.
A further object of the present invention is to avoid the problems and disadvantages encountered in prior art methods for providing a compression force for electrical and thermal connections.
Yet another object of the present invention is to allow the integration of several different types of electronic components in one package called a power bar without the need for special precautions to achieve uniform thermal and electrical contacts.
Still a further object of the present invention is to provide packaging for wafers which avoids a "hockey puck" configuration so that the electronic packages can be used in applications where size and weight are minimal.